If you are going to buy used, just be sure your seller has a rock-solid reputation. If you don't know your seller, or you don't have a way to investigate the reputation of the seller, it would be safer to buy new.
Used lenses can not only have fungus, as stated earlier, but also dust or scratches that could affect the quality of your photos.
That said, I have bought used lenses in the past and they have been fine, but I always made sure that I could trust the seller. It is worth the extra money to make sure you have a lens that has not been damaged.
Note: the passage of time alone will not affect the sharpness of the lens, but it has to be taken care of appropriately.
2.8 Macro, Canon 15 2.8 Fisheye, Canon 35 2.0, Canon 85 1.8...